Best bet is to contact NEC's tech support at 800-NEC-INFO. There may be
something peculiar about the discs you are trying to run.
You _could_ switch the drive to single speed mode, but it would need a special
program to do so… and I haven't seen any!
The NEC-38 has a DIP switch on the bottom. Read the manual for setting standard
speed. It also switches between SCSI and SCSI-2, which may also be the problem.